Output leading zeros on numbers as seen in input (Fixes #550)

This fixes #550 and some other related issues!

To do this I had to add a status bool to `Number` to preserve the state that we have parsed!
Turned out to be trickier than anticipated, but it should now be in line with ruby sass!

I initially implemented that status flag on all number representations down to the C interface, but decided to drop it for now as it would mean a small change in the API and I [added this a local branch] [1] for now.

[1]: https://github.com/mgreter/libsass/tree/issue/550-c-interface
Nope, it should be that way, as node sass outputs it with a leading zero if it is comming from a variable (even in you've written it without). This is exactly what makes libsass behave the same way and the spec tests include this one!